For many years it was assumed that the oceans could take all the waste that was dumped into them. This was true up to a certain extent… until the turn of the last century when marine activity was limited. As the population increased, industrialization and economic activities also increased, including marine activities like fishing, shipping, oil drilling, development of ports and harbors, passenger travel, and exploitation of the sea bed for minerals. These activities consequently generated waste grew to such an extent that marine pollution become extremely evident.
The first legislation against marine pollution was enacted by United Kingdom in 1923 and was known as the “Oil in Navigable Water Act." Oil had become a major source of energy replacing coal and it was not only transported in larger volumes, but it also found favor as the fuel for marine diesel engines as well. It has led to increased marine pollution just as the transportation of oil as a cargo led to waste oils being generated as residues in tanks after discharging of cargo (as well as from the main engine).
The residues in the tank were discharged into the sea either during cleaning operations or when ballasting the tanks and discharging the “dirty ballast" into the sea before loading again.
At one point of time it was estimated that about 500,000 barrels of oil were being discharged into the sea. A new method for reducing the pollution was adopted called “Load-on-Top." In this method every oil tanker will be provided with a slop tank having a capacity of between 0.8 and 3 percent of the oil carrying capacity of the ship.
After cleaning the cargo tanks, dirty ballast residues and tank washing are transferred to this slop tank where they are allowed to stand for an adequate period without turbulence. The oil and water separates out and after determination of the oil/water interface by a detector, the water is pumped out from the bottom in compliance with regulations. The remaining oil residues are transferred to the shore reception facility or, if the new cargo is compatible, it can be loaded on top of this residue.